1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a high-frequency semiconductor device for use in microwave communication, millimeter wave communication, and the like.
2. Related Art
In recent years, wireless communication devices that have rapidly become widespread, such as mobile telephones, tend to utilize microwave waveband or millimeter waveband. To process such high-frequency signals, research and development are being made of various techniques in the field of semiconductors. As one example of such techniques, a high-frequency device incorporates a matching circuit for getting the performance of an active element included in the device. Like this combination of an active element and a matching circuit, the MMIC (MonolithicMicrowave IC) technique is an example for integrating active elements and passive elements into one semiconductor device.
According to the MMIC technique, active elements and passive elements are formed on the same chip. Here, using for example a GaAs substrate with a high resistivity of about several tens Mxcexa9xc2x7cm can lower loss of passive elements (a spiral inductor, a transmission line, etc.). On the other hand, the use of a silicon substrate, which is cheaper than a GaAs substrate, cannot lower loss of passive elements because the silicon substrate has a low resistivity.
FIG. 1 is a graph showing the relationship between a resistivity (xcexa9xc2x7cm) of a substrate and a loss (dB/m) per unit length of an Au wire, when the Au wire with a thickness of 3 xcexcm and a width of 70 xcexcm is laid on an oxide film with a thickness of 0.2 xcexcm formed on the substrate. It should be noted here that the Au wire constitutes a transmission line with a resistivity of 50 xcexa9.
As FIG. 1 shows, a line loss varies depending on a frequency of a signal applied on the line. A lowering rate of the line loss is substantially saturated when the resistivity is around 100 kxcexa9xc2x7cm. With the resistivity being raised above this value, the line loss is not drastically lowered any more. On the other hand, according to the single-crystal silicon formation technique presently being available, a resistivity of a silicon substrate has its maximum at about several kxcexa9xc2x7cm, and so the present technique fails to form a silicon substrate having a resistivity higher than several kxcexa9xc2x7cm.
In view of this difficulty, a semiconductor device disclosed in Japanese published unexamined application No. 2000-232212 is proposed as one example. FIG. 2 is a cross sectional view of the semiconductor device relating to the disclosure. As FIG. 2 shows, the semiconductor device 1 includes a silicon substrate 101 having a high resistivity achieved by diffusing Au therein. On the top surface of the silicon substrate 101, an oxide film 102 is formed.
An SOI (Silicon On Insulator) layer 103 made of single-crystal silicon is embedded in the oxide film 102. A source region 104 and a drain region 105 are provided each adjacent to the SOI layer 103. The source region 104 and the drain region 105 are formed by impurities injected into the oxide film 102. On the SOI layer 103, a gate insulation film 106 is formed. Within the gate insulation film 106, a gate electrode 107 is formed. In the remaining region on the oxide film 102, an interlayer insulation film 108 is formed.
Above the source region 104 and the drain region 105, contact holes 109 and 110 are respectively formed so as to pierce the inter layer insulation film 108. The contact holes 109 and 110 are filled with tungsten 111 and 112, to provide interlayer connection wires. On the interlayer insulation film 108, aluminum wires 113 and 114 are laid. These aluminum wires 113 and 114 are electrically connected to the source region 104 and the drain region 105 respectively, via the interlayer connection wires.
The interlayer insulation film 108, and the aluminum wires 113 and 114 are covered with a wire protection film 115, to prevent short circuit and the like. The wire protection film 115 is, for example, a nitride film, an oxide film, or the like. In this semiconductor device 1, a resistivity of the silicon substrate 101 is raised with the above-mentioned use of Au being diffused therein. Therefore, loss of passive elements can be lowered even though an expensive GaAs substrate is not used.
In the semiconductor device 1, however, a countermeasure should be taken for preventing diffusion of Au atoms from the silicon substrate 101 into the SOI layer 103. For this purpose, the oxide film 102 needs to be formed considerably thick, for example as thick as 2 xcexcm. Forming such a thick oxide film increases the manufacturing cost, and therefore, the above conventional technique can be considered unpractical.
Also, in the semiconductor device 1, passive elements and active elements are both formed on the silicon substrate 101. This means that any defect occurring in an active element causes the entire semiconductor device including the passive element part to be rejected as defective, thereby degrading the manufacturing yield.
Further, passive elements, in particular a spiral inductor, occupy a large area of the substrate when being mounted onto the substrate. Considering this, the substrate with conforming passive elements being rejected only due to a defective active element is by no means favorable.
In view of the above problems, the object of the present invention is to provide a high-frequency semiconductor device at low cost and with high manufacturing yield, while lowering loss of passive elements.
The above object of the present invention can be achieved by a semiconductor device including: a silicon substrate that contains at least one of Au, Pt, and Cu in a state of being diffused, and on which a first circuit element is formed without a heating process; and a semiconductor chip in which a second circuit element is formed by a heating process, the semiconductor chip being flip-chip mounted to the silicon substrate.
According to this structure, such a case where the silicon substrate is heated due to a heating process for forming the active elements can be avoided. Therefore, diffusion of Au atoms present in the silicon substrate into other parts of the semiconductor device, in particular into the active elements, can be avoided. Therefore, a thick oxide film employed by the above conventional technique does not need to be provided, contributing to decreasing cost of the substrate and to downsizing the device.